Electric actuating units in vehicles, for example sliding roofs or window raising mechanisms, are equipped with position determining means. These position determining means are necessary to allow the respective window or sliding roof to stop precisely in a predefined position. Such position determining means are also necessary to satisfy statutory requirements relating to protection against trapping.
With known position determining means in window raising mechanisms a first initialization takes place during production by moving the window to its upper mechanical stop. This is detected by a control unit and used as a reference for subsequent position counting operations. These position counting operations when the window is being raised and lowered involve counting Hall sensor pulses associated with the rotation of the respective drive motor. These pulses result from the rotation of the motor shaft, to which a magnetic wheel is secured, which is provided with alternating sectors or poles of different polarity in a circumferential direction.
Two Hall sensors with a 90° offset are generally used for position counting. It is thus possible to determine both the speed of rotation and the direction of rotation.
To reduce the costs of the drive, it is already known that just one Hall sensor can be used for position counting. This is only provided to count the pulses. Information relating to the direction of rotation is derived from the known states of the motor control relay. In this instance however position cannot be determined precisely during a reversing operation after trapping has been detected, as mechanical inertia means that the actual direction of rotation of the motor does not always correspond to the direction of rotation provided for by the instantaneous control signals of the switching relay. A position counting error therefore results. To ensure correct position counting, particularly in respect of statutory requirements relating to protection against trapping, it is however necessary for position determination to be extremely accurate.
If two Hall sensors are used for position counting, to avoid the problem of position inaccuracy, higher system costs are incurred, particularly in the motor and cable harness.
To avoid the problem of position inaccuracy it is already known that the position count can be reinitialized regularly by moving the window to its upper mechanical stop after every window movement or every nth window movement. However this has the disadvantage of placing greater strain on the mechanical system, which in turn incurs higher costs for the mechanical system. And it is not always possible. For example if the window is moved frequently and frequent trapping takes place without the window being closed or opened completely, post-initialization is not possible.
A method and device for determining the actual reversal of the direction of rotation of a following rotary drive are already known from EP 1 175 598 B1. With this method an asymmetrical rotor-side sensor wheel is used to supply pulse sequences that are proportional to speed with interim reference pulses. These pulses are detected by a single stator-side sensor and evaluated in an evaluation apparatus. This evaluation apparatus determines the actual reversal of the direction of rotation after the direction of rotation has been switched, in particular after a polarity reversal of an electric rotary drive motor, by taking into account a pulse signal correction value derived from the pulse signal count in each instance between a reference signal before and after the actual reversal of the direction of rotation that can be ascertained from a change from an instantaneous increase to an instantaneous decrease in pulse signal lengths after the direction of rotation has been switched.
A further device for determining the actual reversal of the direction of rotation of a reversing rotary drive is known from DE 10 2005 047 366 A1. This device also uses a sensor wheel with a coding structure that is configured asymmetrically in respect of distribution along the circumference of the sensor wheel and a single detector, which generates a pulse signal as a function of rotor speed by scanning the coding structure during rotation of the sensor wheel. This pulse signal is supplied to an evaluation unit, which determines the actual reversal of the direction of rotation by evaluating the pulse edges. The above-mentioned coding structure of the sensor wheel is formed by coding sectors of a first sector width and a reference coding sector pair having a second sector width.
The disadvantages of the devices described in EP 1 175 598 B1 and DE 10 2005 047 366 A1 are that an asymmetrical sensor wheel is required, only small deviations can be corrected, stable motor operation is required for correction and synchronization is lost during reversing.